A biphenyl thiosemicarbazide based fluorogenic chemosensor for selective recognition of Cd2+: application in cell bioimaging

Abstract

A diversified biphenyl thiosemicarbazide based chemosensor (HBMC) has been fabricated and reported for the specific detection of Cd²⁺ in a MeOH : H₂O (4 : 1) solution. We observed a chromogenic change from colorless to light yellow colour, and it showed a “turn-on” fluorogenic change from non fluorescent to blooming cyan colour. In fluorometric titration a sharp “turn-on” emission for Cd²⁺ was observed with a ∼16 fold increase in fluorescence intensity value at 496 nm by incremental addition of Cd²⁺ ions in the MeOH : H₂O (4 : 1) solution. The reversibility of the chemosensor (HBMC) was confirmed by a sequential addition of the EDTA solution. Again the binding stoichiometry of HBMC with Cd²⁺ was found to be 2 : 1, as confirmed by Job's plot analysis and HRMS spectra of the HBMC–Cd²⁺ complex. The mechanism for Cd²⁺ sensing in MeOH : H₂O (4 : 1) is based upon the inhibition of CN isomerization and ESIPT process and simultaneously turning on the CHEF (chelation enhanced fluorescence) process. The limit of detection for Cd²⁺ was found to be in the order of 10⁻⁸ (M), which implies that HBMC is an efficient probe to detect Cd²⁺ at the microscopic level. A reusability study was performed and on-sight detection of cadmium ions by the chemosensor (HBMC) was established by dip-stick experiment. In vitro detection of Cd²⁺ in human breast cancer cells (MDA-MB-231) by HBMC discloses its cell permeability and biocompatible nature. Computational studies (DFT and TDDFT) with the probe HBMC and HBMC–Cd²⁺ complex were also performed.